<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        //1、在浏览器中输出内容
        document.write("Hello World!");
        //2、控制台打印内容
        console.log("Hello World!");
        //3、弹出警告框
        alert("Hello World!");

        //变量的定义
        let a=20
        
        
    </script>
</body>
</html>